Many power plants operate during conditions of partial loads or variable heat input, and several methods are known how to continuously produce power despite a change in the heat input.
In one method, a partial admission turbine is employed whereby motive fluid is admitted over only a selected circumferential distance of the annular area available at the turbine blade inlet. A decrease in turbine efficiency results since only a portion of the turbine blades is filled with motive fluid although the entire portion of the rotating blades is subject to frictional losses. Also, added costs are involved due to the need of a plurality of injection valves in order to ensure the partial admission and due to the need to reinforce the turbine blades as a result of the harsh load conditions, i.e. variable pressure for each rotation.
In another method, a turbine injection valve is throttled to control the mass flow rate of motive fluid admitted to the turbine. However, the internal efficiency of the turbine is reduced during a partial load due to the pressure drop and irreversibility of the injection valve during throttling. Also, the stages following the inlet stage suffer from inefficiencies.
At times, variable nozzles are employed; however, they are complex and are associated with leakage losses and maintenance problems.
In a third method, the thermal efficiency of a power plant is maintained by employing a regenerative cycle whereby condensate is pumped around the turbine casing, counterflow to the direction of the flow of the motive fluid being expanded within the turbine while heat is being transferred thereto. Due to the cost of the additional equipment, including valves, pumps and control devices, and of construction work to provide extraction ports on the turbine casing, a power plant employing a regenerative cycle is uneconomical and is implemented only in very large power plants, e.g. having a capacity of 100-1000 MW.
In a fourth method, the boiler temperature or pressure is controlled as a function of the variable load or the variable heat input. Thermal efficiency of the power plant is reduced because of the lower temperature.
The present invention provides an apparatus and method for improving power plant efficiency at partial loads or reduced heat input which are not subject to thermodynamic losses as a result of reduced heat input.
Additionally, the present invention provides an apparatus and method for improving power plant efficiency at partial loads or reduced heat input without suffering from losses associated with throttling or partial admission.
Furthermore, the present invention provides an apparatus and method for improving power plant efficiency at partial loads or reduced heat input without the complexity of regenerative cycles.
Other advantages of the invention will become apparent as the description proceeds.